Grasses (Poaceae) are an abundant and widespread source of food, feed and energy. Non-edible agricultural waste residues as well as dedicated energy crops are crucial lignocellulosic feedstocks for sustainable, low-carbon biorefining into fuels, chemicals and other products. The innate recalcitrance of plant cell walls to deconstruction poses major limitation for efficient and cost-efficient biorefining.
The aim of this PhD thesis was to address if modification of mineral nutrition might be a prospective strategy to alter cell wall recalcitrance. The study focused on nitrogen (N), an essential element that promotes plant growth and productivity. An additional focal point was silicon (Si), a beneficial element which improves plant performance and alleviates biotic and abiotic stresses, but may increase the recalcitrance of biomass to degradation. The impacts of Si, inorganic N form and supply rate on plant performance and cell wall composition were investigated in the grass model plant species Brachypodium distachyon and in the crop species wheat.
Field-grown wheat was subjected to three rates of N ranging from 50 to 150 kg N ha-1. The plants were harvested at different stages of maturation and defined parts of the straw were analyzed for cell wall characteristics relevant for further biomass processing. The straw N concentration corresponded with the level of N input, but the yield of straw biomass was not significantly affected. Increased N supply resulted in higher abundance of arabinogalactan proteins and lignin, and a decrease in pectin methylesterification. The level of N supply and the accompanying cell wall alterations did not affect the saccharification efficiency. On the other hand, different forms of inorganic N significantly affected the composition of cell walls of Brachypodium distachyon. Nitrate promoted cellulose and inhibited lignin synthesis at the heading stage of growth, but the differences between the treatments disappeared as the plants matured. Sole ammonium supply resulted in growth reduction and premature senescence along with higher concentration of lignin and mixed linkage glucans. Pectin concentration and composition were particularly affected by treatment with various N forms. At the heading stage, plants supplied with nitrate showed the lowest methylesterification degree of pectins while ammonium-fed plants contained relatively more pectin rhamnogalacturonan-I with a higher abundance of arabinan side chains.
Brachypodium distachyon wild type and low-silicon 1 (Bdlsi1-1) mutant plants served as tools to characterize Si deposition sites and effects on plant performance and biomass properties. The low-silicon 1 mutant (Bdlsi1-1) plants carried a mutation in the Si influx transporter BdLSI1 localized in the roots which resulted in more than
90% reduction in shoot Si content. Low shoot Si was associated with a significant reduction of seed weight and changes in macrohair morphology and patterning, thus emphasizing the importance of Si for successful development in grasses. At the cell wall level, several alterations in non-cellulosic polysaccharides and lignin were observed in vegetative organs of the mutant plants. These alterations, however, did not affect the recalcitrance of the straw towards enzymatic hydrolysis. Thus, the saccharification efficiency of the straw was not modified. With respect to seed composition, the abundance of mixed linkage glucans was severely reduced in the mutant, resulting in lower glucose, but higher xylose, release during enzymatic saccharification.
High N supply negatively affected the Si concentration in shoots of the wild type Brachypodium distachyon plants, possibly through modulation of the silicon transporter BdLSI1. On the other hand, a high N supply increased the partitioning of Si to spikelets in the Bdlsi1-1 mutant plants, resulting in comparable Si concentration in spikelets of mutant and wild type plants.
It is concluded that Brachypodium distachyon is a suitable model plant for studies of the importance of nutrient supply for the cell wall composition in grasses. Furthermore, the level and form of nitrogen supply affect the composition of cell walls in lignocellulosic biomass. The same is the case for changes in silicon uptake and deposition. However, the accompanying effects on cell wall recalcitrance towards enzymatic hydrolysis are negligible.